1. Field of the Invention
The present invention relates to a projection optical system, and more particular to a projection optical system capable of effectively eliminating stray light.
2. Description of the Prior Art
The digital micromirror device (DMD) is an image display device. The DMD is usually used in a conventional projection optical system for adjusting an illumination light from an illumination light source. The DMD comprises a plurality of micromirrors, and it is controlled by an inputted signal to rotate, so as to reflect the illumination light in three separate directions. Accordingly, the projection optical system can provide an image with high resolution.
Referring to FIG. 1, FIG. 1 is a schematic diagram of the projection optical system 10 of the prior art. In the projection optical system 10, the DMD 14 comprises a plurality of micromirrors arranged in an array thereon. Each of the micromirrors is controlled by an inputted signal to rotate respectively. When a micromirror is rotated to +12 degree, it is in an ON state; when a micromirror is rotated to 0 degree, it is in a FLAT state, and when a micromirror is rotated to −12 degree, it is in an OFF state. In the DMD 14, those micromirrors in the ON state reflect the illumination light shone thereon to generate a signal light 22; those micromirrors in the FLAT state reflect the illumination light shone thereon to generate a first stray light 18, and those micromirrors in the OFF state reflect the illumination light shone thereon to generate a second stray light 20. If the above stray lights 18 and 20 are projected with the signal light 22 into the projection lens, the luminance of the signal light will be seriously lost, such that the contrast of projected image will be worse, and the resolution of the image will be influenced.
In the projection optical system 10 of the prior art, a block sheet 16 is disposed between a projection lens 12 and a DMD 14 to shield off the second stray light 20. However, the height of the block sheet 16 of the prior art can only fully shield off the second stray light 20 and cannot shield off the first stray light 18, so the first stray light 18 will directly be projected into the projection lens 12, causing worse contrast of the image. The present solution for the above problem is to elongate the block sheet 16 to shield off the first stray light 18. However, there are areas where the first stray light 18 and the signal light 22 overlap, so when the block sheet 16 is elongated to fully shield off the first stray light 18, a portion of the signal light 22 will be also shielded off. The contrast of the projected image will be worse, and the resolution of the image will be influenced.
With the development of science and technology, the demand on high image resolution is higher and higher. Since the block sheet of the prior art cannot fully shield off the stray light, such that the image resolution is influenced, the present invention provides a projection optical system capable of effectively eliminating the stray light to solve the above-mentioned problems.